1 | /*
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2 | * Copyright 2014-2016 The OpenSSL Project Authors. All Rights Reserved.
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3 | *
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4 | * Licensed under the OpenSSL license (the "License"). You may not use
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5 | * this file except in compliance with the License. You can obtain a copy
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6 | * in the file LICENSE in the source distribution or at
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7 | * https://www.openssl.org/source/license.html
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8 | */
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9 |
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10 | #ifndef HEADER_CONSTANT_TIME_LOCL_H
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11 | # define HEADER_CONSTANT_TIME_LOCL_H
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12 |
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13 | # include <openssl/e_os2.h> /* For 'ossl_inline' */
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14 |
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15 | #ifdef __cplusplus
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16 | extern "C" {
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17 | #endif
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18 |
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19 | /*-
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20 | * The boolean methods return a bitmask of all ones (0xff...f) for true
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21 | * and 0 for false. This is useful for choosing a value based on the result
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22 | * of a conditional in constant time. For example,
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23 | *
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24 | * if (a < b) {
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25 | * c = a;
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26 | * } else {
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27 | * c = b;
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28 | * }
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29 | *
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30 | * can be written as
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31 | *
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32 | * unsigned int lt = constant_time_lt(a, b);
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33 | * c = constant_time_select(lt, a, b);
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34 | */
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35 |
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36 | /*
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37 | * Returns the given value with the MSB copied to all the other
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38 | * bits. Uses the fact that arithmetic shift shifts-in the sign bit.
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39 | * However, this is not ensured by the C standard so you may need to
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40 | * replace this with something else on odd CPUs.
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41 | */
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42 | static ossl_inline unsigned int constant_time_msb(unsigned int a);
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43 |
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44 | /*
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45 | * Returns 0xff..f if a < b and 0 otherwise.
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46 | */
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47 | static ossl_inline unsigned int constant_time_lt(unsigned int a,
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48 | unsigned int b);
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49 | /* Convenience method for getting an 8-bit mask. */
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50 | static ossl_inline unsigned char constant_time_lt_8(unsigned int a,
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51 | unsigned int b);
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52 |
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53 | /*
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54 | * Returns 0xff..f if a >= b and 0 otherwise.
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55 | */
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56 | static ossl_inline unsigned int constant_time_ge(unsigned int a,
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57 | unsigned int b);
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58 | /* Convenience method for getting an 8-bit mask. */
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59 | static ossl_inline unsigned char constant_time_ge_8(unsigned int a,
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60 | unsigned int b);
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61 |
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62 | /*
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63 | * Returns 0xff..f if a == 0 and 0 otherwise.
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64 | */
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65 | static ossl_inline unsigned int constant_time_is_zero(unsigned int a);
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66 | /* Convenience method for getting an 8-bit mask. */
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67 | static ossl_inline unsigned char constant_time_is_zero_8(unsigned int a);
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68 |
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69 | /*
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70 | * Returns 0xff..f if a == b and 0 otherwise.
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71 | */
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72 | static ossl_inline unsigned int constant_time_eq(unsigned int a,
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73 | unsigned int b);
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74 | /* Convenience method for getting an 8-bit mask. */
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75 | static ossl_inline unsigned char constant_time_eq_8(unsigned int a,
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76 | unsigned int b);
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77 | /* Signed integers. */
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78 | static ossl_inline unsigned int constant_time_eq_int(int a, int b);
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79 | /* Convenience method for getting an 8-bit mask. */
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80 | static ossl_inline unsigned char constant_time_eq_int_8(int a, int b);
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81 |
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82 | /*-
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83 | * Returns (mask & a) | (~mask & b).
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84 | *
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85 | * When |mask| is all 1s or all 0s (as returned by the methods above),
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86 | * the select methods return either |a| (if |mask| is nonzero) or |b|
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87 | * (if |mask| is zero).
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88 | */
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89 | static ossl_inline unsigned int constant_time_select(unsigned int mask,
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90 | unsigned int a,
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91 | unsigned int b);
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92 | /* Convenience method for unsigned chars. */
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93 | static ossl_inline unsigned char constant_time_select_8(unsigned char mask,
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94 | unsigned char a,
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95 | unsigned char b);
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96 | /* Convenience method for signed integers. */
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97 | static ossl_inline int constant_time_select_int(unsigned int mask, int a,
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98 | int b);
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99 |
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100 | static ossl_inline unsigned int constant_time_msb(unsigned int a)
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101 | {
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102 | return 0 - (a >> (sizeof(a) * 8 - 1));
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103 | }
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104 |
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105 | static ossl_inline unsigned int constant_time_lt(unsigned int a,
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106 | unsigned int b)
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107 | {
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108 | return constant_time_msb(a ^ ((a ^ b) | ((a - b) ^ b)));
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109 | }
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110 |
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111 | static ossl_inline unsigned char constant_time_lt_8(unsigned int a,
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112 | unsigned int b)
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113 | {
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114 | return (unsigned char)(constant_time_lt(a, b));
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115 | }
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116 |
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117 | static ossl_inline unsigned int constant_time_ge(unsigned int a,
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118 | unsigned int b)
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119 | {
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120 | return ~constant_time_lt(a, b);
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121 | }
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122 |
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123 | static ossl_inline unsigned char constant_time_ge_8(unsigned int a,
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124 | unsigned int b)
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125 | {
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126 | return (unsigned char)(constant_time_ge(a, b));
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127 | }
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128 |
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129 | static ossl_inline unsigned int constant_time_is_zero(unsigned int a)
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130 | {
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131 | return constant_time_msb(~a & (a - 1));
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132 | }
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133 |
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134 | static ossl_inline unsigned char constant_time_is_zero_8(unsigned int a)
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135 | {
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136 | return (unsigned char)(constant_time_is_zero(a));
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137 | }
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138 |
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139 | static ossl_inline unsigned int constant_time_eq(unsigned int a,
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140 | unsigned int b)
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141 | {
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142 | return constant_time_is_zero(a ^ b);
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143 | }
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144 |
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145 | static ossl_inline unsigned char constant_time_eq_8(unsigned int a,
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146 | unsigned int b)
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147 | {
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148 | return (unsigned char)(constant_time_eq(a, b));
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149 | }
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150 |
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151 | static ossl_inline unsigned int constant_time_eq_int(int a, int b)
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152 | {
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153 | return constant_time_eq((unsigned)(a), (unsigned)(b));
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154 | }
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155 |
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156 | static ossl_inline unsigned char constant_time_eq_int_8(int a, int b)
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157 | {
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158 | return constant_time_eq_8((unsigned)(a), (unsigned)(b));
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159 | }
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160 |
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161 | static ossl_inline unsigned int constant_time_select(unsigned int mask,
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162 | unsigned int a,
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163 | unsigned int b)
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164 | {
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165 | return (mask & a) | (~mask & b);
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166 | }
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167 |
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168 | static ossl_inline unsigned char constant_time_select_8(unsigned char mask,
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169 | unsigned char a,
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170 | unsigned char b)
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171 | {
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172 | return (unsigned char)(constant_time_select(mask, a, b));
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173 | }
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174 |
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175 | static ossl_inline int constant_time_select_int(unsigned int mask, int a,
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176 | int b)
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177 | {
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178 | return (int)(constant_time_select(mask, (unsigned)(a), (unsigned)(b)));
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179 | }
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180 |
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181 | #ifdef __cplusplus
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182 | }
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183 | #endif
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184 |
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185 | #endif /* HEADER_CONSTANT_TIME_LOCL_H */
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